A couple walks today in Philadelphia, where the weather service called a snowstorm “life-threatening” and warned people to “shelter in place.”

NEW YORK — A powerful nor’easter pounded the mid-Atlantic and the Northeast early today, prompting flight cancellations, school closures and warnings from city and state officials to stay off the roads.

The National Weather Service issued blizzard warnings for parts of Pennsylvania, New Jersey, New York, Connecticut, Rhode Island, Massachusetts, New Hampshire, Maine and Vermont.

The storm was expected to dump 12 to 18 inches of snow on the New York City metro area with wind gusts of up to 55 mph.

The weather service’s office near Philadelphia called the storm “life-threatening” and warned people to “shelter in place.” Coastal flood warnings were in effect from Massachusetts to Delaware.

According to the airline-tracking website FlightAware, more than 5,000 flights today were canceled, including more than 2,800 in the New York City area, where about 200 passengers were stranded at John F. Kennedy airport. Amtrak also canceled and modified service up and down the Northeast Corridor. In New York City, the above-ground portions of the subway system were being shut down today.

In the nation’s capital, the U.S. Office of Personnel Management announced a three-hour delayed arrival for non-emergency employees at area federal offices — with an option of unscheduled leave or telework. Emergency employees of the federal government in the Washington, D.C., area were to report on time unless otherwise directed.

New York Gov. Andrew Cuomo a state of emergency today for all of New York’s 62 counties, including New York City’s five boroughs. The Democrat also directed non-essential state employees to stay home from work as authorities advised people to stay off the roads.

In Massachusetts, where forecasts called for 12 to 18 inches of snow, Gov. Charlie Baker encouraged motorists to stay off the roads and to take public transit only if absolutely necessary. The fast snowfall rates will “create hazardous driving conditions across the Commonwealth,” he said.

Schools in New York City, Philadelphia, Boston and elsewhere were all closed today.